Help with final drive

[pilotprice]

Hi guys

Looking for some help with my caterham sigma 150 5 speed final drive

Today we went on a brilliant blat round north wales, I took my sigma 150 and my mates had a k series 1600 and a westy

I swapped cars with the K owner and couldn't believe the acceleration differences and in gear pick up of his car compared to mine? He said he car was ex academy 1.6l built by Mcmillian. (It felt about 160hp, but the owner said it was 115?)

The car just felt much much eager to go and I noticed his box always has the rpm higher for the same speeds than mine does

For example

80mph 5th gear mine revs at 3400
His car would be at 4050?

We both have a 5 speed so should have the same 3.92 diff? I'm suspect re my diff as the previous owner was very sketchy over it when I comented that it looked recently painted, he simply said its a standard ford diff

The k owner has 13 inch wheels and I have the 15s

My car seems to struggle with its top end as the engine can't get on cam in 5th

Thoughts?

Edited by pilotprice on Saturday 30th July 18:56

[pilotprice]

Also the K had significantly less drivetrain movement, does the k have an extra mount over the sigmas 2?

[battered]

Jack it up and count wheel turns per rotation of the propshaft.

Also, if you have 15s, does this increase the rolling circumference?

Finally, k series cars had a standard Type 9. The ratios are well known on these, but some people have them modified with a higher top and lower 5th, which makes them MUCH better and nearly as good as a 6 speed. You can check your ratios by jacking up the rear wheels and comparing crank rotations (disable the ignition, obviously, and turn it slowly with a spanner) with gearbox output shaft rotations.

[pilotprice]

Ok cool, cheers for that

So what numbers am i looking at for wheel rotations when turning the prop

[battered]

3.92 to 1, I think you said. That's how a diff ratio works.

[pilotprice]

Great, thanks

ill do this tomorrow

[ghibbett]

pilotprice said:

80mph 5th gear mine revs at 3400
His car would be at 4050?

We both have a 5 speed so should have the same 3.92 diff? I'm suspect re my diff as the previous owner was very sketchy over it when I comented that it looked recently painted, he simply said its a standard ford diff

I would suspect you don't have a 3.92 final drive. I have a Sigma 140 (Supersport) with the 3.92, and 80 mph in 5th is definitely >4000rpm.

It would also account for the difference in acceleration. I couldn't believe how much of a difference driving a 3.62 vs 3.92 final drive made.

[Eccles52]

I'd agree wit ghibbett above, I went from a 3.9 to a 3.6 thinking it would be better for touring but all it did was take the edge off the acceleration - now back with the 3.9
The 13" are smaller than the 15" in diameter (if running 60 profile 13" tyres) so that will also make a difference.
Another thing would be if the K car has a lightened flywheel which will also make it rev more freely.

Ivan

[pilotprice]

I'm going to jack up the rear and get count my wheel revs whilst turning the prop to see what I have, I suspect I have a 3.6!

PS I have a lightweight flywheel on mine too